This invention relates generally to towed vehicle brake controllers and, in particular, to a novel towed vehicle brake controller which can be retrofitted to a tow vehicle to effect a gradual application of the towed vehicle brakes proportional to the movement of the tow vehicle brake pedal.
Towed vehicles, such as recreational and utility trailers adapted to be towed by automobiles and small trucks, are commonly provided with electronic braking systems. The electric brakes generally include a pair of brake shoes located at each wheel which, when actuated, frictionally engage a brake drum. An electromagnet is mounted on one end of a lever to actuate the brake shoe, and is drawn against the rotating brake drum when an electric current is applied, thereby pivoting a lever to actuate the brake shoes. Typically, the braking force provided is proportional to the electric current applied to the electromagnet. The electric current may run as high as 12 amperes on a double axle trailer.
The first electric brake controllers for actuating towed vehicle brakes incorporated a large rheostat switch mounted in a position for the driver of the tow vehicle to be able to manually activate the towed vehicle brakes as needed. Later designs employed a hydraulic slave cylinder in the controller with a hydraulic line connecting the tow vehicle brake system to the controller. Thus, when the vehicle brakes were applied, the controller sent an electrical current to the towed vehicle brakes in proportion to the pressure applied to the tow vehicle braking system. Alterations to tow vehicle hydraulic braking systems where generally discouraged by automobile manufacturers, resulting in the introduction of electronic brake controllers which did not connect directly to the tow vehicle hydraulic systems. These electronic controllers were simply timers which applied the towed vehicle brakes at a pre-set time interval after the application of the tow vehicle brakes. The "timer" type controllers did not sense or accommodate the difference between gradual brake application and an emergency stop. That is, when the tow vehicle brake is applied, the towed vehicle brakes were applied after the pre-set time interval, regardless of the braking conditions.
An improvement over the timer-type controllers was the addition of a mercury switch which employed the inertia of small amount of liquid mercury to close a pair of contacts during rapid deceleration of the tow vehicle, triggering full application of the towed vehicle brakes during an emergency stop.
Still further improvements over the timer-type controllers consisted of the incorporation of a pendulum or similar device to sense deceleration of the tow vehicle caused by braking. An electronic circuit would generate a brake control signal proportional to the pendulum displacement during deceleration. These designs suffer from several inherent problems. First, because the towing vehicle and towed vehicle are connected together, the driver must initially slow the towed vehicle by application of the tow vehicle brakes. The towing vehicle must undergo sufficient deceleration for the pendulum or similar device to activate the towed vehicle brakes. If the controller is fine-tune adjusted, and a heavy towing vehicle is pulling a lightweight towed vehicle, the deceleration sensor system works well. However, very few drivers are capable of adjusting these controllers with the degree of precision necessary for optimal performance. In the more common situation, where a light tow vehicle is pulling a heavy towed vehicle, it is impossible to produce maximum towed vehicle braking by setting the control so that it activates with emergency-type stopping power even in normal, non-emergency stopping situations. Essentially, the momentum of the heavy towed vehicle will "push" the tow vehicle, preventing it from decelerating at a sufficient rate to fully activate the towed vehicle brakes.
Regardless of the type of deceleration sensor or brake control signal initiator device, known electronic brake controllers also usually include an analog pulse width modulator which receives the brake control signal from the sensing unit. The pulse width modulator is responsive to the brake control signal for generating an output signal comprising a fixed frequency pulse train. The pulse width modulator varies the duty cycle of the pulse train in proportion to the magnitude of the brake control signal, thus the duty cycle of the pulse train corresponds to the amount of towed vehicle braking desired. The output of the pulse width modulator is typically used to control the switching of power transistors on and off, supplying power to the towed vehicle brakes, with the resulting brake application directly proportional to the duty cycle of the pulse width modulator output.